Environmental Engineering Reference
In-Depth Information
structure of the resultant scale are primarily governed by the alloy composition
and the reaction mechanism under the prevailing conditions. Since the alloy oxi-
dation behavior can be extremely complex, it is necessary to break down the field
into various limiting cases, applicable specifically to binary alloys but in a more
general way to commercial alloys, which can be treated quantitatively or at least
semiquantitatively.
Let us consider the oxidation of a single-phase alloy, A-B, in a single oxidant
system, where B is the less noble metal. For simplicity it is assumed that the two
metals form only one oxide of each, i.e., AO and BO. So for steady-state scale
growth on alloy A-B, the following cases may arise:
1.
Compositions near pure A, where AO is produced almost exclusively, at
least in the external scale
2.
At sufficiently high concentrations of B, where BO is formed exclusively
3.
At intermediate composition range, where both AO and BO are formed
In the intermediate composition range (3), depending on the properties of AO
and BO, the two oxides may be completely miscible, producing an oxide solid
solution; or they may be completely immiscible, producing multiphase scales;
or they may combine to form another complex oxide compound, e.g., ABO
2
.In
addition, internal oxidation may also take place. For ternary and multicomponent
alloys, the degree of complexity increases further.
The schematic classification of the scale morphologies according to distribu-
tion of phases in the scale, obtained by oxidizing or sulfidizing alloys, was pre-
sented for the first time by Moreau and B´nard [24]. Subsequently, this was
adopted by Wood [25] and Wallwork [26]. Dalvi et al. [27] attempted to rational-
ize the complex processes that occur during the oxidation of binary alloys utiliz-
ing ternary diffusion theory, in particular the concept of a diffusion path on a
ternary phase diagram, like A-B-O or A-B-S. Such analysis provides scope not
only for theoretical predictions of the alloying elements likely to be preferentially
oxidized but also the expected steady-state scale. They can also supplement infor-
mation on subsequent reactions, such as the changes occurring when the internal
oxide is incorporated in the main scale. Such theoretical analysis is based on the
assumptions that a local thermodynamic equilibrium prevails at the alloy-scale
and scale-gas interfaces and that the oxide scales are free of cracks and pores,
and adherent to the alloy substrate. More recently, Bastow et al. [28], by consider-
ing the elemental distributions through the scales, presented a more comprehen-
sive classification of the various scale morphologies for the oxidation and sulfi-
dation of binary alloys, complementary to that of the diffusion path approach of
Dalvi et al. [27]. The application of thermodynamic concepts, though extremely
useful, is used sparingly in the present discussion; more emphasis is placed on
factors affecting the spatial distribution of components in the scale and alloy,
